Bow and stern thrusters are a common piece of equipment on many boats. They provide the necessary thrust to change the yaw direction of a boat in tight quarters. In many instances the presence of wind or current in a tight marina makes safe handling of some boats nearly impossible without a thruster.
However, conventional thrusters come at a price. They are expensive to purchase and very expensive to install. Typically, the boat hull most be modified to add a “tunnel” through the boat where the thruster propeller is located and the thrusted water flows. Power must be routed to the thruster to provide the high current requirements. Finally, control devices and wiring must be installed from the helm to the bow thruster. Since much of this work is below the water line, the boat must be hauled out of the water.
The tunnel and thruster components add to the hydraulic drag of the boat. To reduce this drag the hulls are often further modified to fair the hull leading into the tunnel opening.
The resulting configuration creates new opportunities for hull leaks and exposes the thruster components to corrosion, fouling from marine life, and catching seaweed and other floating debris. Since the thruster components are underwater and inaccessible they are difficult to clean and maintain without hauling the boat out of the water periodically.
There are existing alternatives to the common tunnel thruster just described. These alternatives address the key issue of having to cut holes in the hull of the boat for the tunnel, but not the other issues. Further, they create additional problems.
One alternative is to mount a “pod” to the bottom of the keel. This pod has a small tunnel through it with a propeller in the tunnel. This eliminates the need for cutting holes in the hull of the boat for placing a thruster tunnel, although it is usually required to modify the hull to fair the pod with the hull. The boat must still be hauled out of the water to drill the mounting holes for the pod, and run the power and control wires through the hull. Furthermore, the thruster is still subject to corrosion and fouling and must be hauled out periodically for cleaning. Importantly, the pod creates more drag than the traditional tunnel approach and renders the thruster vulnerable to damage from strikes from floating debris (such as logs) or from groundings.
A second alternative is to bolt a thruster unit to the front of the bow below the water line. This also eliminates the need for cutting holes in the hull of the boat for placing a thruster tunnel, but it does not address the other issues raised. The boat still needs to be hauled out of the water to drill the underwater mounting holes and to run the power and control cables. This second alternative is not very popular due to the sizeable drag created by its externally bolted structure, and the vulnerability to damage from floating debris, ground strikes, and trailering.
As useful as bow and stern thrusters are, the negatives prevent many boat owners from outfitting them on their boats. This is particularly true of sailboat owners who cannot afford the increased drag created by conventional bow thrusters.